1. Field of the Invention
This invention relates to a sputtering apparatus for forming thin films in vacuum and, more particularly, to a high-speed low-temperature sputtering apparatus used for forming, on a mass production scale, recording films of magnetic disks or optical magnetic disks or Sendust or Permalloy films as yokes of magnetic heads or thin film magnetic heads by using a target of a magnetic material.
2. Prior Art
Sputtering apparatus of the magnetron discharge type, in which a magnetic field normal to an electric field is provided in the neighborhood of the target surface to trap electrons and produce high density plasma, is thought to be capable of producing a film having a desired thickness quickly and on a mass production basis, and their development is in progress.
Where the magnetic field is fixed for sputtering, the target is locally eroded, and the utility factor is inferior. Accordingly, a system, in which a magnetic field is rotated during sputtering, would provide uniform erosion of the target surface and thus improve the utility factor and extend the life of the target.
FIGS. 1 and 2 are views for explaining target erosion in the prior art. FIG. 1 is a plan view showing a disk-like target, and FIG. 2 is a section taken along line C --C in FIG. 1. The Figures illustrate only a portion of prior art magnetron discharge type sputtering apparatus including the target. In the Figures, a disk-like target 81 has its surface 82 subjected to sputtering a vacuum. Magnetic field application unit 84 is provided on the side a back surface 83 of target 81. It is supported on a turntable 85 for rotation with shaft 86 about center of rotation 87. Magnetic field application unit 84 includes permanent central and other magnets 841 and 842 and yoke 840. The pole surface of central magnet 841 and that of outer magnet 842 are of opposite polarities to each other. Outer magnet 842 is frame-like and surrounds central magnet 841 as will be understood from areas 841 and 842 defined by dashed lines in FIG. 1.
FIG. 1 is a plan view showing the shape of erosion area 88 produced in target 81 in case where the magnetic field is held stationary during sputtering.
Here, erosion area means an entire sputtered area having 5% or more of the maximum erosion depth. Erosion area 88 based on the magnetic field set up by the illustrated magnet array as noted above is substantially rectangular, as shown in the shaded area. A broken line indicates a maximum erosion depth portion 89 of erosion area 88 resulting from quickest erosion. Usually, this portion is substantially linear as is well known in the art. This maximum erosion depth portion is referred to as the quickest erosion portion. The position of the quickest erosion portion 89 can be accurately determined by experiments. However, it can be roughly estimated from the distribution of the magnetic field produced by magnetic field application unit 84.
In the above prior art apparatus, the magnetic field is rotated by operating turntable 85. FIGS. 3 and 4 are views for explaining target erosion in cases where the magnetic field is rotated during sputtering of target 81. As shown in the Figures, erosion area 98 defined by two circles concentric with the center of rotation 87 of the magnetic field (shown shaped in FIG. 4) is formed in the entire area where the field is rotated.
The rate of erosion is not uniform over the surface of target 81. Particularly, there is a strong trend to form a circular groove 99 by strong erosion at a position close to center of rotation 87 of the magnetic field in broad erosion area 98 between concentric circles noted above under rotating magnetic field. If such a circular groove 99 is to be formed in an initial stage of sputtering, this portion of circular groove 99 is subjected to a stronger magnetic field than the field formed in the other sputtering area. Therefore, this portion of groove 99 is subject to acceleratedly quick erosion compared to the rest of the sputtering area, thus increasing the non-uniformity of erosion.
This phenomenon is particularly pronounced where a ferromagnetic target is used. In this case, only the local circular groove 99 is strongly eroded even if the magnetic field is rotated. That is, the magnetic field is rotated. Without effect, resulting in inferior target utility factor. Besides, in this case deterioration of film formation performance on a workpiece substrate (such as film formation rate, film thickness distribution and step cover factor) is inevitable.
There are various proposals of providing for uniform target erosion in order to improve the target utility factor and to solve the problems noted above. Typically, there is a technique disclosed in Japanese Patent Disclosure No. Sho 62-60866. The principles underlying the disclosed technique are as follows. Erosion rate under a stationary magnetic field as shown in FIG. 1 will be considered. Center of rotation of the magnetic field, if caused, is taken as reference point. It is assumed that n plasma groups exist on a circle concentric with the center of rotation 87 and with radius r. It is also assumed that plasma in the same plasma group provides a uniform erosion rate.
The erosion rate provided by plasma is different with different plasma groups. The higher the plasma density is, the higher the erosion rate is. The erosion rate provided by plasma in an i-th plasma group that is found on a circle of radius r is denoted by R.sub.ri (.ANG./min.) (i=1, 2, . . . , n). It is assumed that plasma groups having the same erosion rate R.sub.ri (.ANG./min.) are found in a circle over a length corresponding to a circumferential angle .theta..sub.ri (rad.) (.theta..sub.ri being the ratio of the circumferential length of plasma providing the same erosion rate R.sub.ri on circle having radius r to distance r from the center of rotation). When the magnetic field is rotated, the erosion rate D.sub.r of various parts of the target on the circle of radius r is the average value of erosion rate R.sub.ri noted above and is given as ##EQU1##
Therefore, the following two conditions A and B may be considered.
Condition A: If the afore-mentioned erosion rate R.sub.ri under a stationary magnetic field takes a constant value R.sub.ri on all points on the circle with radius r and zero in the other area (condition A), we have ##EQU2## This equation can be written as ##EQU3## where L.sub.ri is the length of arc subtending each angle .theta..sub.ri.
Condition B: Although R.sub.r generally varies with r, if R.sub.r has a constant value R for each radius r, i.e., irrespective of the radius r (condition B), we may have an equation ##EQU4## This equation can be rewritten as ##EQU5##
In the disclosed technique noted above, both conditions A and B noted above are satisfied substantially over the entire target surface by providing a plurality of magnets having the same shape on the back side of the target in such an arrangement that the term ##EQU6## and hence ##EQU7## is constant over the entire target surface, thus ensuring uniformity of erosion. In other words, according to this prior art technique erosion is caused likewise and to the same depth over the entire sputtering area of target under a rotating magnetic field. When this is obtained, uniformity of erosion on the target surface can be ensured, and the utility factor of the target can be increased.
In other words, this prior art technique permits improvement of the utility factor of the target when it is used for high-speed low-temperature sputtering using a magnetic target to form, on a mass production scale, large area recording films of magnetic disks and or optical magnetic disks, Sendust or Permalloy films as yokes of magnetic heads or thin film magnetic heads. In this case, however, the thickness of the film deposited on the substrate fluctuates by 5% or above, that is, the thickness distribution of the film is insufficient.
FIG. 10 shows experimental data of the thickness distribution of a film formed on a substrate. In the graph, the abscissa corresponds to the substrate position, and the ordinate corresponds to the film thickness shown in a normalized form with the maximum thickness taken as unity. Dashed plot A represents an example of thickness distribution of a film obtained with the prior art apparatus described above. It will be seen that the maximum-to-minimum thickness ratio is about 1:0.8.